Separation units, such as atmospheric distillation units, vacuum distillation units, and product strippers, are major processing units in a refinery. Atmospheric or vacuum distillation units separate crude oil into fractions according to boiling point so downstream processing units, such as catalytic cracking, hydrogen treating or reforming units, will have feedstocks that meet particular specifications. Crude oil separation is accomplished by fractionating the total crude oil at essentially atmospheric pressure and then feeding a bottoms stream of high boiling hydrocarbons, also known as topped crude or long resid, from the atmospheric distillation unit to a second distillation unit operating at a reduced pressure (vacuum).
The vacuum distillation unit typically separates the atmospheric unit bottoms into gas oil vapors based on boiling point, including light vacuum gas oil, heavy vacuum gas oil, lube oil distillates, and vacuum reduced crude. The non-distillable residual fraction from the vacuum tower also known as “vac resid” or “short resid”, leaves the vacuum distillation unit as a heavy, viscous, liquid bottoms stream. The bottoms fraction separated from crude oil in the atmospheric distillation unit is fed to a flash zone in the lower portion of the vacuum tower. Although the heavier hydrocarbons do not vaporize, they may be carried into the lighter hydrocarbons due to entrainment in the vapors which ascend into the rectification zone above the flash zone of the tower. The entrained heavier hydrocarbons are typically contaminated with metals, such as vanadium or nickel, which can poison the downstream catalytic processing, such as hydrotreating, hydrocracking, or fluid catalytic cracking.
If the entrainment of the heavier components can be significantly reduced or eliminated, a significant improvement in the quality of the gas oil product can be realized in both yield and quantity with consequential improvements in the feed for hydroconversion units, catalytic cracking units, as well as in gas oil distillates, or lube oil distillates.
Various methods of reducing entrainment of residuum from the flash zone have been developed. Many distillation towers, especially petroleum vacuum towers, use feed inlet devices, usually known as feed horns, for introducing the feed stream to the flash zone. One type of feed horn uses a tangential entry for the vapor-liquid feed that opens into a peripheral open bottomed horn. The horn can be an annular or arcuate channel defined by an outer peripheral wall defined by the inside wall of the tower shell and an internal arcuate wall spaced from the peripheral wall of the tower with a closed top. The stream of heated feed enters the horn from the inlet and the liquid and vapor components pass along the channel between the walls while the liquid and vapor components separate from centrifugal force since the force on the denser liquid is substantially greater than the force acting on the vapor. The separated liquid flows downward due to gravity into the lower portion of the tower and eventually towards the stripping zone for collection at the bottom of the tower. The vapor component also flows downwardly and then out of the horn into the lower pressure flash zone and is swept upwardly through the hollow central core of the tower towards the wash zone.
One example of a peripheral horn is shown in U.S. Pat. No. 4,770,747 which, in this case, has angularly disposed vanes connected between the walls of the channel so that vapor-liquid separation takes place evenly along the arc length of the horn. Another example is shown in U.S. Pat. No. 4,315,815 in which corrugated vanes are disposed in the horn for utilizing the centrifugal motion to create turbulence in the stream in the inlet horn. In this case, the turbulence causes a portion of the fine particle size bituminous material to impinge on the surfaces of the inlet horn and recombine with the fluid so that vaporized solvent and steam can be withdrawn.
U.S. Pat. No. 5,516,465 and U.S. Pat. No. 5,605,654 describe feed horns which are intended to increase the horizontal distribution of the vapor so that enhanced vapor/liquid interaction in the tower above the feed zone is achieved. One form of vapor distributor comprises an annular wall which is spaced inwardly from the shell of the tower to form a feed channel and which is built up from a number of circumferential segments which are radially staggered to form outlet ports along the circumference of the wall. Each outlet allows a portion of the vapor stream flowing through the distributor to leave the channel and flow into the central core of the tower.
Another form of feed horn is shown in U.S. Pat. No. 6,889,961; this uses guide vanes in the annular channel which extend across the channel and upwards through its open bottom to direct the mixed phase feed in a downward direction into the open core of the tower. The vanes are positioned at gradually increasing heights in the direction of stream flow in the channel so that they progressively pare off a portion of the total liquid/vapor flow in the channel and direct it into the lower portion of the tower.
The problem with the known feed horn devices is that they still allow an amount of vapor with entrained liquid to move up through the core of the tower into the upper wash zone. In a typical tower, it will be found that the overflash, which includes vacuum gas oils from the wash oil, which will be collected in the overflash collection tray, typically has a high percentage of resid and is not suitable for certain feed applications, especially for fluid catalytic cracking (FCC). In order to use the overflash more effectively, especially for FCC feed, it is desirable to reduce the entrainment of resid further. Additionally, more effective de-entrainment will improve the reliability of the wash zone. Excessive resid entrainment in the wash zone accelerates formation of coke, forcing sub optimal operation and premature shutdown. Higher quality overflash, such as would be acceptable for FCC, can increase wash oil rates and virtually eliminate the risk of coking, allowing units to operate at higher temperatures and higher efficiencies.
Experience and testing has shown that the conventional types of feed horn such as those described in the patents noted above inherently direct vapor into the zone below the inlet device, increasing the flow strength or creating localized interference with liquid collected on the wall of the flash zone above the stripping section. Thus, there is a need for a separation device in which entrainment of liquid resid by the vapor flow can be significantly reduced.